THE collar bone or clavicle is long and nearly straight and acts as a strut, preventing the shoulders from passing inwards towards the midline. In some of the very rare cases where these bones are absent the two shoulders, if pulled forwards, can be made to meet together in the front of the chest. The inner end of the clavicle articulates with the upper end of the breast bone which has already been mentioned, while its outer end is in contact with the shoulder blade or scapula. This bone is flat and lies on the back of the upper part of the thorax. Although there is no joint between it and the chest wall, it is capable of moving upwards or downwards and it can slide forwards, thus enormously increasing the mobility of the shoulder. At the outer end of the scapula is a shallow cup-shaped depression covered with cartilage, which forms a joint with the humerus (the bone which forms the upper part of the arm).
The shoulder joint is of the ball-and-socket variety, but unlike the hip joint, where great stability is desirable and the cup is exceedingly deep, here the cup is shallow in order to increase the possible movement. Of course this means that the joint is relatively weak and in practice it is the joint which is most frequently dislocated. However, Nature has been very careful, for above the joint are two strong, bony processes bridged by a thick ligament which prevents the arm being raised from the side of the body to more than a right angle, for it is this movement that causes dislocation.
We all know, however, that we can raise our arms above our heads, so this movement must come from somewhere else besides the joint. Actually it is possible because the shoulder blade can rotate on the chest wall. Here again, therefore, Nature has so contrived matters that a maximum amount of movement can take place with the least possibility of damage being done.